Computer users have long had a need for continuous, non-disrupted operation. Therefore, methods have been developed to modify and update computer programs concurrently with their execution with new code (a patch) that is loaded into the computer.
Typically, a computer program consists of various sections such as executable machine code, static data variables, and temporary data. The executable code may comprise various procedures that are called via their address in memory. A static data variable is kept valid in the same memory address during the entire execution of the program. In contrast, a temporary data variable (and its location in memory) is only valid during certain periods of the program execution (e.g., while a specific procedure executes).
A patch to computer program code replaces either parts of or the entire computer program code. Methods that replace only parts of computer program code are described, e.g., in U.S. Pat. No. 5,321,844, European Patent No. EP 0 492 251 B1, and European Patent No. EP 0 757 314 A1.
The main processor firmware in existing IBM® eServer® zSeries® systems may be patched concurrently so that the complete computer program code is replaced. Such method assumes that it is possible to replace the currently running code with new code at a time where the temporary data are irrelevant with respect to the program execution. The method permits preservation of the static variables and their content.
The concurrent patch operation is executed by a concurrent loader process that runs as a background task. The concurrent loader loads new computer program code (i.e., code load) into the computer system memory and prepares the code for execution. Once the loading and preparation is completed, the concurrent loader sets the computer program to be patched into a state where temporary data can be ignored during the concurrent patch operation. Finally, it switches from the original code to the new code in an atomic operation. This entire procedure is referred to as the application of a concurrent patch.
The preparation of the new program code for its execution consists in resolving and adapting all address references in the code load to the addresses of the memory section into which the code is loaded. This step performed by the concurrent loader is known as relocation. For generating the program code, a standard linker program may be used that does not need special knowledge about the concurrent patch procedure. Thus, the concurrent patch application is transparent to the programmer; there is no need to know how the concurrent patch application functions when implementing the program. In fact, there is no difference between a code load used for a concurrent patch application and a code load that can be loaded by a loader that is not a concurrent loader.
The format of the computer program code and the format of the code load used for a concurrent patch is the standard Executable and Linking Format (ELF). Any linker program that supports the ELF format may be used. A code load in the ELF format may be used for a concurrent patch, and it may be loaded by any compliant loader that supports the ELF format (not necessarily a concurrent loader).
Function pointers generated by a compiler cannot be handled transparently by a concurrent loader alone. Accordingly, an extension is necessary. A function pointer is an element of many high-level programming languages (e.g., C and C++) that may be used instead of a procedure name literal string in order to refer to a specific procedure. Function pointers allow algorithms to use procedures as manipulation objects.
Usually, function pointers are translated into the address of the referenced procedure by the programming language compiler. The content of a data variable may be a function pointer. Since static data variables are preserved during the concurrent patch application, static data variables containing the address of a procedure are preserved as well. However, there is no guarantee that the address of the referenced procedure is still the same after the concurrent patch application. Between the assignment of an address of a procedure to a function pointer and the actual usage of the function pointer, one or more concurrent patch operations could have changed the address of the procedure. Thus, the function pointer does not necessarily point to the correct address of a procedure after application of a concurrent patch.
An address points to a memory location but does not provide any additional information, and the content stored in such memory location cannot be identified to be the address of a procedure, a data variable, a pure number, or even an instruction of the processor. A procedure is translated into a sequence of processor instructions by the compiler. A given sequence of processor instructions cannot be related to a procedure later on.
U.S. Pat. No. 5,481,713 and U.S. Pat. No. 5,938,766 disclose non-concurrent patch methods for replacing only parts of computer program code. These methods support function pointers. The function pointers are stored in a special memory area called a vector table. The vector table is maintained by a loader program that is responsible for the patch application.
The background of applying patches to a computer program concurrently with its execution is disclosed in U.S. Publication No. 2006/0242491. A further development of this method is disclosed in U.S. Publication No. 2007/0006201, which provides a method and a system for generating and applying a monolithic concurrent patch. Full support for function pointers is provided that is transparent to the programmer and nearly transparent to the concurrent loader. A reference to a function pointer is translated into a sequence of processor instructions called a function descriptor instead of translating it into an address. The purpose of the function descriptor is to jump to the memory location of the sequence of instructions generated by the compiler for the procedure referenced by the function pointer. The function descriptor is masked as a static data variable and therefore preserved during the application of a concurrent patch. The address of the jump to the procedure is updated by the regular relocation process during application of a concurrent patch.
However, the method described by U.S. Publication No. 2007/0006201 does not describe a method to allow a safe removal of function pointers within a concurrent patch. Thus, it is not possible to apply a concurrent patch with a code load via the method described by this publication in a scenario in which one or more of the functions pointers have been removed.
Moreover, known methods and systems do not allow application of a concurrent patch to a computer program that is organized in modules; rather, known methods and systems only allow application of a concurrent patch to one monolithic code load.
Furthermore, with known methods and systems, it is not possible to change the size of existing variables. Rather, known methods and systems merely provide for the addition of new variables and the deletion of existing variables.